There are many uses for thin films carried on rigid or flexible substrates. Printed circuits are exemplary of such utility and are widely used. Other characteristics can be built into thin films by depositing them in certain patterns.
Typically, patterned thin films have been prepared by depositing the film on a substrate using a deposition mask or by etching unpatterned thin films using conventional photoresist and etching. These techniques have a variety of disadvantages. Deposition masks must be precisely positioned and held in place where low/critical tolerances are involved. It is difficult to avoid relative movement between the mask and the deposition substrate. This is particularly true where the substrate is flexible.
Etching techniques also have disadvantages. Use of this technique involves depositing relatively large areas of an unpatterned thin film and etching it to the desired pattern. The deposition of larger areas of a continuous film produces stress within the film and promotes cracking and spalling. Stress problems are accentuated where multiple layers of distinct materials having different thermal and/or chemical properties are involved. Moreover, etching is not as precisely controllable a process as may be necessary for small-dimension, tight tolerance patterns. Further, many materials are difficult to etch by ordinary chemical techniques. That is, their etch rate is impractically slow with common etchants or such severe etch conditions are required that surrounding materials are undesirably affected.
As discussed above, the generation of patterns in inorganic films is known and has found wide spread use in the printed circuit industry. The process that is widely used is photolithography. A variation on the conventional photolithography process is to use a negative relief mask. Negative relief masks are also used in electroforming and in additive photolithography, but in these instances the top of the mask is not coated. The use of a negative relief mask in vacuum metallization processes results in the top of the mask being coated. Solvents attack the relief mask either through pin holes in the film or through discontinuities in the film along the edges of the mask or by cutting both the mask and the film so as to provide direct access of the solvent to the mask. In some limited instances, inorganic negative relief masks have been used to take advantage of the higher temperature capabilities and lower vapor pressure in vacuum metallization chambers. When using inorganic negative deposition masks, the masks are normally removed by selective etching with an acid To date, processing has been limited to small rigid substrates and only single film thicknesses about 0.1 .mu.m thick have been imaged.